There has conventionally been a machine tool, for example, where a work is supported by a rotatable main spindle and machined while a tool is being fed to the work. However, in the conventional machine, when a cutting depth in cutting machining is made larger than necessary, so-called “chatter vibration” is generated during the machining, resulting in deterioration in finishing accuracy of a machined surface. Particularly, “regenerative type chatter vibration” which is self-excited vibration generated during machining. To suppress the “regenerative type chatter vibration”, it is known as described in Patent documents 1 and 2 to use a rotation speed which is equal to a value derived by obtaining a natural frequency of a system such as the tool or work where the “chatter vibration” is generated, or a frequency of the chatter vibration during the machining; multiplying the natural frequency or the chatter frequency by 60; and dividing the multiplied frequency by the number of tool blades and a predetermined integer. The rotation speed calculated based on the above procedure is hereinafter referred to as a stable rotation speed.
In order to obtain the natural frequency of the system where the “chatter vibration” is generated, there is known a method in which, for example, the “natural frequency” is obtained by applying impulse excitation to the tool or work to measure a frequency, as described in Patent document 1. Further, in order to obtain the chatter frequency during the machining, for example, a sound sensor is arranged near the tool or work in rotation, and the sensor detects a vibrational frequency during the machining to obtain the “chatter frequency”, as described in Patent document 2.    [Patent document 1] Japanese Unexamined Patent Publication No. 2003-340627    [Patent document 2] Japanese Translation of PCT No. 2001-517557